1. FIELD OF THE INVENTION
The invention relates to fuel assemblies for nuclear reactors of the type comprising a skeleton having two end pieces connected together by elongate elements such as guide tubes and grids spaced apart along the guide tubes and forming cells for holding a bundle of fuel elements in position at the nodes of a regular lattice. It is particularly suitable for use in spectral shift light water reactors in which the initial reactivity excess is compensated by undermoderating the reactor and hardening the neutron energy spectrum. An increase fraction of the neutrons is then absorbed by fertile material. In such a reactor, it is possible to increase the moderation rate for a predetermined fuel burn-up by removing rods which contain neutron transparent or fertile material (depleted uranium, for example) from guide tubes which are then invaded by water.
2. PRIOR ART
In fuel assemblies of the above-defined type, the grids fulfill a number of separate functions. They guide and support the fuel elements containing fissile material. They provide resistance to transverse shocks which the assemblies may undergo during handling, accidents and, possibly, seisms. In most cases, the grids cause turbulence in the coolant flow and deflect coolant streams within the assembly to homogenize the temperature and avoid the formation of hot spots which could lead to local boiling and to sheath failure. A description of such fuel assemblies may be found in numerous reference, for instance European No. 54,236 (Westinghouse) and U.S. Pat. No. 4,059,483 (Anthony).
The grids required for fulfilling all these functions impress on the flow of the coolant in the assembly a pressure drop which should be reduced as much as possible. But, concurrently, research work carried out for further increasing the thermal performances of nuclear reactors have led to increasing the coolant flow rate and, therefore, the forces exerted by the coolant on the components of the assemblies. The problem becomes more acute in spectral shift reactors since part of the cross-section should be reserved for guide tubes associated with water displacer rods and the lattice pitch should be decreased for locating the same amount of fissile material in the fuel assembly. As a result, the cross-sectional area available for coolant flow is reduced, which causes an increased pressure drop and a reduction of the protection margin before nucleated boiling in the top part (i.e., the downstream part) of the assemblies.